The present invention relates to an optically activated switch for high power RF signals. More specifically, but without limitation thereto, the present invention relates to photovoltaic switches having relatively low activation power.
Optically-activated RF switches include photoconductive switches, phototransistors, surface-depleted optical FET's, and photovoltaic field effect transistors. A photoconductive switch typically has an on-resistance that is linearly proportional to the power of the optical control signal, thus a low on-resistance of about 2 ohms typically requires an optical control signal power of over 50 mW. Switching time is generally linearly proportional to the carrier lifetime of the photoconductor and inversely proportional to the on-resistance. A high speed photoconductive switch thus tends to have the disadvantage of a high on-resistance. Phototransistors generally require high optical control power for a low on-resistance, and consequently suffer from the problem of a low off-resistance. The load resistance of a typical RF application, however, such as a dipole antenna, is approximately 73 ohms. An RF switch capable of switching a 100 W signal for such applications should have an off-resistance of at least 10 k ohms to adequately isolate the input from the output in the off state.
U.S. Pat. No. 4,712,020 issued on Dec. 8, 1987 to Basile discloses a single PIN diode as a switching element with photovoltaic cells in a bias circuit, but suffers the disadvantage of requiring a reverse bias dependent upon the input signal amplitude to switch the PIN diode to the off state (col. 4, ln. 21-24).
A need therefore exists for an RF switch having a low on-resistance, a high off-resistance, and no elecrical control signal power.